This is my strategy. I set my review batch size at three and I use Reorder to choose which kinds of review I do.
When I level up I always do the radical lessons if they’re there, and I might pad the final batch of radicals out with some kanji. Radicals are almost free reviews anyway. At most twelve lessons per session (in one hour) with a few sessions a day, getting smaller over the course of the day.
Once the radicals are out of the way, I do batches of just vocab with maybe a single batch of kanji to keep things interesting - so nine vocab lessons and three kanji lessons, or twelve vocab lessons and three kanji lessons. I do this until the vocab is all gone, because once I start levelling kanji I don’t want to have to deal with any vocab lying around.
Once all the vocab lessons are out of the queue, I get to just focus on doing kanji lessons for a few days. Those are good days.
Usually by the time the last of the kanji are out of my lesson queue - three or six kanji per session, with a couple of sessions every day - the guru reviews are starting to come in and I’m starting to amass vocab lessons again.
I don’t get any rest days doing levels this way but it is efficient without being overwhelming. It just requires consistency.
Generally speaking I don’t do lessons for any more than fifteen items at a time. If it’s just kanji, the most I like to do is six. Three is more normal. I do a few lesson sessions a day but if I start dropping too many Apprentice 2 reviews, I go slower. If it’s all coming easy, sometimes I push my luck and go quicker.
On the later levels, fifteen lesson items in a session is plenty. Twelve is more normal. I can still level quickly enough but there’s no gap day while I’m waiting for the Guru reviews. There’s always lessons coming in every day as long as I keep getting things right. It’s like a marathon pace - it requires consistency but you’re never really sprinting.
For Level 60 I will ignore all that and just scoff the entire lesson queue in a single day like I used to. That’s how I want to celebrate getting to level 60 and I don’t care how much of it I get wrong. I have lots of pet leeches already.